for DX1, DX2 being non empty set
for F1 being Function of DX1,REAL
for F2 being Function of DX2,REAL
for G being Function of [:DX1,DX2:],REAL
for Y1 being non empty finite Subset of DX1
for p1 being FinSequence of DX1 st p1 is one-to-one & rng p1 = Y1 holds
for p2 being FinSequence of DX2
for p3 being FinSequence of [:DX1,DX2:]
for Y2 being non empty finite Subset of DX2
for Y3 being finite Subset of [:DX1,DX2:] st p2 is one-to-one & rng p2 = Y2 & p3 is one-to-one & rng p3 = Y3 & Y3 = [:Y1,Y2:] & ( for x, y being set st x in Y1 & y in Y2 holds
G . (x,y) = (F1 . x) * (F2 . y) ) holds
Sum (Func_Seq (G,p3)) = (Sum (Func_Seq (F1,p1))) * (Sum (Func_Seq (F2,p2)))